1. Field of the Invention
The present invention is relates to a flat CRT (Cathode Ray Tube), in particular to a flat tension mask type CRT which improves structures of a shadow mask and a panel.
2. Description of the Prior Art
In general, a cathode ray tube is for reproducing a received electric signal into a video signal, it is used for a video displaying apparatus such as a television, a monitor etc., and it is divided into a curved surface type and a flat type.
Between the above-mentioned CRT, the flat CRT which is superior than the curved CRT in the image distortion and outer light reflexibility aspects will now be described.
As depicted in FIG. 1, a general flat CRT 1 comprises a panel 13, a funnel 14, and an electron gun 17.
The panel 13 comprises a fluorescent screen 11 constructed with red, green, blue stripes or dots shape fluorescent material, and a safety glass 12 adhered to the front surface by a resin as a hardening adhesion for preventing damage of the panel 13 due to durability lowering caused by the flat surface of the panel 13.
In addition, a shadow mask 18 is fixedly formed on a rail 15 adhered to the back of the fluorescent screen 11 of the panel 13 by welding.
And, the funnel 14 as a vacuum bulb is fixedly formed on the rear of the panel 13, the electron gun 17 for scanning an electron beam 16 is placed in a neck unit 14a of the funnel 14, and a deflection yoke 21 for deflecting the electron beam 16 is installed on the outer circumference of the neck unit 14a. 
Particularly, as depicted in FIG. 2, the shadow mask 18 is fabricated as a very thin film having a thickness of about 25 xcexcm in order to improve a transmittance of the electron beam 16, and a plurality of through holes 19 are formed with a certain interval.
The plurality of through holes 19 are formed by an etching process in fabrication of the shadow mask 18, as depicted in FIG. 3, an outlet size L0 of the through hole 19 on the front surface is bigger than an inlet size L1 of the through hole 19 on the inner surface.
Meanwhile, a vertical direction width between the through holes 19 on the shadow mask 18 is called as a bridge width W.
A video reproducing process of the above-described flat CRT will now be described.
As depicted in FIG. 1, the received electric signal is controlled, accelerated, collimated by a voltage applied to each electrode, and its orbit is adjusted to a horizontal direction or a vertical direction by a magnetic field of the deflection yoke 21.
After that, as depicted in FIG. 3, the deflected electron beam 16 radiates the fluorescent material of the fluorescent screen 11 coated on the back of the panel 13 by passing through the through hole 19 of the shadow mask 18, accordingly a picture is reproduced.
However, in the above-mentioned shadow mask 18 used for the conventional flat CRT, a rolling process is additionally required in order to fabricate the shadow mask 18 as a very thin film having a thickness of about 25 xcexcm, deformation of the mask largely occurs in the etching process for forming the through hole 19, an error such as a fracture etc. occurs in fabrication process of the flat CRT (Cathode Ray Tube), accordingly fabrication of the shadow mask is not easy, error rate in fabrication is high, handling is difficult, and the price is high.
In addition, in the conventional flat CRT, when the shadow mask 19 is fixed to the rail 15, a rip etc. can occur due to a shear stress, because the heat quantity of the shadow mask 18 is small, a doming can occur due to collision of the electron beam and the electron beam can not land accurately on the fluorescent material, accordingly luminance and color vividness of the picture quality can lower.
Because of the above-mentioned problems, as depicted in FIG. 4, when the thickness t of the shadow mask 18 increases from (a) to (b), a size L1xe2x80x2 of the through hole on the inner surface can be fabricated as same with the size L1 which is the size when the shadow mask 18 has a thickness t of 25 xcexcm in fabrication of a through hole 19xe2x80x2 of a shadow mask 18xe2x80x2 with the etching process.
However, in the shadow mask 18xe2x80x2, because of a problem from the fabrication technology point of view, a size L0xe2x80x2 of the through hole 19xe2x80x2 on the front surface has to be a little smaller than the size L0 when the shadow mask 18 has a thickness t of 25 xcexcm.
Accordingly, in the shadow mask 18xe2x80x2 having the increased thickness txe2x80x2, the inner surface 19axe2x80x2 from the inlet to the outlet of the through hole 19xe2x80x2 is formed gently, and the bridge width Wxe2x80x2 between the through holes 19xe2x80x2 increases.
In order to improve the durability of the shadow mask 18, when the thickness t increases, because the outlet size L0xe2x80x2 of the through hole 19xe2x80x2 decreases due to a difficult point of the fabrication process of the through hole 19xe2x80x2, the transmittance of the electron beam decreases.
It can be described as below table 1.
As described in table 1, when the shadow mask 18xe2x80x2 has a thickness of not less than 50 xcexcm, because the outlet size L0xe2x80x2 of the through hole 19xe2x80x2 decreases, the bridge width Wxe2x80x2 increases. Herein, the electron beam transmittance of the shadow mask 18xe2x80x2 decreases not less than 10%.
After all, when the thickness t increases in order to improve the durability of the shadow mask 18, the electron beam transmittance decreases, the ratio of the electron beam 16 for radiating the red, green, blue fluorescent material of the fluorescent screen 11 decreases, accordingly the luminance as the brightness of the CRT lowers.
In order to solve above-mentioned problems, the object of the present invention is to provide a flat CRT (Cathode Ray Tube) which is capable of making fabrication and handling of a shadow mask easier by designing a panel so as to get an appropriate luminance while increasing a thickness of the shadow mask, and improving a picture quality by heightening the final luminance of the CRT.
In order to achieve the object of the present invention, the flat tension mask type CRT in accordance with the present invention comprises a flat panel fixed to the front of a funnel, and a shadow mask fixed inside of the funnel with a certain distance from the panel, herein the shadow mask has a thickness of 50 xcexcmxcx9c80 xcexcm, and the panel has a transmittance of 47%xcx9c50%.
The flat tension mask type CRT has a luminance of not less than 31 FL, and the panel has a thickness of 13 mmxcx9c14.5 mm.